[Collins] 75A2 / 75A3 mixer modification

[Jim Brannigan]

Cal etal,
It may have been from the '55 QST article.  I have it around somewhere, but 
have not been able to locate it.

I did the mod in the early 70's so my memory is kinda hazy.....
I also did not have the test equipment (or knowledge) I have today.
If I recall, my 75A3 would not hear the first RF stage noise.  I thot that 
changing the mixers would help, it did not.
Ultimately, I traced the tube noise to the added IF stage that compensates 
for the mechanical filter loss and was never able to resolve it to my 
satisfaction.
That 75A3 was subjected to a lot of abuse by me.  At various times I added a 
product detector (plugged into the FM socket), changed BFO's (crystal, 
variable, internal, external) and a whole host of other changes.
I finally realized that I would never make this receiver into a SSB/CW DX 
killer like my friends 75S-1 and sold it to buy a Yaesu FT-101, it also had 
a terrible receiver. The useless mods to the FT101 are another story.

All the work on the A3 did teach me a valuable lesson DON'T MESS WITH 
COLLINS ENGINEERING!!!!
The A3 is a nice AM radio.  There were good reasons for developing the A4 
and 75S series.
My late WE S-line is my backup to my TT OMNI-VI+ (a great receiver) and the 
R-390A is a great AM RX for my Ranger.

Jim
PS 300+ DX countries on the S-line and two ARRL CW DX NLI section wins, 92 
es 93.



> Jim:
> What is your take on the modification? Good, bad, or
> ugly?
>
> Cal,N6KYR/8
>
> --- Jim Brannigan <jbrannig at optonline.net> wrote:
>
>> I changed both mixers in a 75A3 many moons ago,
>>
>> QST 56,57,58?
>>
>> Jim
>>
>>
>> > On Mon, 2006-06-26 at 18:56 -0700, Glen Zook
>> wrote:
>> >> I had an inquiry about a 75A3 that has had the
>> first
>> >> mixer changed out from the 6BA7 to a 12AT7.
>> Since I
>> >> had not seen this change before I thought that I
>> would
>> >> see if anyone had any information on the
>> modification
>> >> so that I could pass along the information.
>> >>
>> >> Of course the same modification would work on the
>> 75A2.
>> >>
>> >> Glen, K9STH
>> >>
>> > I think that was published in an early 73
>> magazine, though it could have
>> > been a 50s vintage CQ that is in my collection.
>> The 12AT7 has far lower
>> > noise but also lower gain. It may take days to
>> find the article, but I
>> > recall it being published.
>> > -- 
>> > 73, Jerry, K0CQ, Technical Advisor to the CRA
>> > All content copyright Dr. Gerald N. Johnson,
>> electrical engineer
>>
>>
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